Relatives of the independent presidential candidate Robert F. Kennedy Jr., are rallying behind his Democratic opponent, President Biden. The political dynasty’s divide mirrors that of the Roosevelt family 100 years ago, a schism that provides hope that whichever side emerges victorious, America wins.

No fewer than 15 Kennedys, gathering Thursday at Philadelphia, threw their support to Mr. Biden, and some made a video endorsement. RFK Jr.’s sister, Kerry Kennedy, spoke for the family, saying that they believe “the best way forward for America is to reelect Joe Biden and Kamala Harris.” Another sister and brother joined her.

“RFK Jr. is on the outs with almost the entire extended Kennedy Clan,” the author of “Coming to Terms with John F. Kennedy,” Stephen F. Knott, told the Sun in February. “They do not see him as an heir to either JFK or RFK.” The Roosevelts faced the same bitter wrangling and found a way to share the mantle.

President Theodore Roosevelt had a junior, too, a Republican like himself, called “Ted” to avoid confusion. President Franklin D. Roosevelt, also a New Yorker but a Democrat, followed the Rough Rider’s path step-by-step in parallel with TR Jr., knowing that to get to the presidency, he had to snatch his cousin’s birthright.

FDR got his chance in 1920, a year after TR’s death, when Democrats chose him as their vice-presidential candidate. TR Jr. called FDR a “maverick” who did not “have the breed of our family.” The Sun wrote of FDR: “The country at large has never before heard of him, as he has never previously figured prominently as a national character.” It endorsed Calvin Coolidge for vice president.

Tim Brady wrote in “His Father’s Son,” a biography of Ted Jr., that many voters “were quick to assume that FDR was just another of the many boys sired by TR.” FDR “was none too quick to clarify.” He invoked TR’s name, adopted his catchphrases like “square deal” and “bully” (as in “good”), and even wore TR-style pince-nez glasses long after they’d gone out of fashion.

The Democratic ticket lost, leaving TR Jr.’s star ascendant and FDR scrambling to bring it crashing back to earth. When TR Jr. won the GOP’s nomination for governor in 1924, fate intervened in the form of President Harding’s Teapot Dome scandal. Although TR Jr. had nothing to do with the scheme to sell federal oil leases, FDR seized his chance.

“In 1924,” the presidential historian David Pietrusza tells the Sun, Eleanor Roosevelt, FDR’s wife and TR’s niece, “drove around with the teapot mounted on her car to tweak TR Jr.,” following him on the campaign trail and helping to sink his chances. Four years later, FDR won New York’s governorship and was nominated for the White House in 1932.

“Franklin is such poor stuff,” Ted Jr. said during the campaign, “it seems improbable that he should be elected president.” When FDR won, his rival was asked just how he was related to the president-elect. “Fifth cousin,” he said, “about to be removed,” meaning as President Hoover’s governor-general in the Philippines.

The split between the Republican Roosevelts of Oyster Bay and the Democrats of Hyde Park, New York, lingered. They disagreed over the New Deal and America First isolationism, until the Pearl Harbor attack made them set aside political differences for the good of the country.

“For all their disagreements with one another,” Mr. Brady wrote, “the cousins evidently maintained a good humor in their personal connections.” Emerging from a meeting with FDR at the White House, TR Jr. — back in uniform as a brigadier general — told the press, “This is our country, our cause, and our president.” Many of the Oyster Bay faction joined him in enlisting.

TR Jr. was the highest-ranking officer and oldest man to go ashore on D-Day. He died a short time after in 1944; FDR followed nine months later. The two Roosevelts had spent decades fighting over TR’s legacy, but in the end, both did credit to it.